(1) Field of the Invention
The present invention relates to protective systems and, particularly, to monitoring the state of operability of a circuit including plural switches which change state in response to the movement of a guard member such as a security door. More specifically, this invention is directed to methods of and apparatus for periodically evaluating the state of operability of a circuit which includes plural, parallel-connected interlock switches. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for employment in monitoring devices which, via a plurality of condition responsive electrical switches, monitor the state, i.e., the open or closed position, of protective coverings such as doors, covers, etc. of machinery or the like. In this demanding operating environment, it is common for plural switches, operatively associated with the secured door, to be hard wired to a remotely located control or alarm. It is possible for the electrical conductors which extend between the switches and the remote control to become damaged over an extended use period of the associated machine. Such damage, i.e., a fault, may take the form of a hard to detect "cross", i.e., a short circuit may develop between a pair of the conductors which are respectively associated with different of the plural interlock switches.
A short circuit of the type discussed above can be laboriously detected by the application of a positive voltage to one of a pair of parallel connected monitoring switches while simultaneously causing the other switch to "switch" zero potential. Observation of the voltage received at the remote control/alarm station during such switching will indicate whether there is a short circuit in the system.
The above-described manual testing technique is, of course, applicable when both of a pair of parallel operated switches operate in the same sense, i.e., are simultaneously either normally closed or normally open. The technique is, however, also applicable in the situation where one of the switches is normally open and the other is normally closed. Under such circumstances, should a short circuit occur, the potentials at monitored terminals located at the control station terminals, which terminals correspond to the output terminals of both switches, will assume the same potential thus indicating the presence of a fault.
The state-of-the-art is also exemplified by the teachings of German Patent 32 41 492. In accordance with this prior art, the switches of the protective circuit to be monitored are connected between a voltage source and a signal processor and the states of the switches are cyclically interrogated through the generation of interrogation pulses which are respectively fed to the processor via a coding line. The processor detects the combination of the pulses on the coding line and thus obtains information as to which switch group as just been interrogated. Although the interrogation of the switching states can be performed in a very short time period, simultaneous monitoring of the switches for their proper functioning is not possible in accordance with the teachings of German Patent 32 41 492.